Mahler started on opening day for the Braves in 1982, when Atlanta won the National League West title. He made four straight opening day starts beginning in 1985. In 1987, he tied an NL record with his third opening day shutout. His best season came in 1984, when he went 13-10 with a 3.12 ERA while hitting .296 at the plate. In 1985, he won his first 7 straight starts and ended up 17-15 with a 3.48 ERA. He pitched twice in the postseason, with the Braves in 1982, and with the 1990 World Series champion Cincinnati Reds. In postseason play, Mahler pitched 3+1⁄3 scoreless innings. Mahler was a key member of the 1990 Reds' pitching staff as a spot starter and reliever, going 7–6 and contributing four saves. He also appeared in ten games with the Montreal Expos in 1991 before returning to Atlanta in mid-season.